Conventionally, communication systems are known in which an originating mobile station and a plurality of mobile stations specified by the originating mobile station (hereinafter, referred to as specified mobile stations) form a group to perform a group call. The group call includes PTT (Push-To-Talk) in which authority to transmit a voice signal, i.e., authority for a user to speak (right to speak), is assigned only to one mobile station that participates in the group call.
Specifically, the group call is performed through a packet switching NW, and is performed by use of SIP (Session Initiation Protocol). The group call is started when each mobile station included in the group logs in to (registers in) an SIP server connected to the packet switching NW (for example, non-patent literature 1).
Thus, the group call cannot be started without a login of each mobile station to the SIP server. Therefore, when the originating mobile station attempts to start the group call, the specified mobile station needs to log in to the SIP server.
Accordingly, for the specified mobile station to log in to the SIP server, information that requests the login to the SIP server (SMS (Short Message Service) information) is transmitted to the specified mobile station through a line switching NW different from the packet switching NW.
Specifically, when receiving information (INVITE) that requests start of the group call from the originating mobile station, the SIP server transmits a request for transmitting the SMS information that requests a login to the SIP server, to an SMS server connected with the line switching NW. Subsequently, the SMS server transmits to each specified mobile station the SMS information that requests a login to the SIP server.
On the other hand, another group call is also known in which a mobile station that has not participated in a group call (hereinafter, referred to as non-participating mobile station) can be invited to participate in the group call halfway. The non-participating mobile station indicates a specified mobile station that does not respond a call from the originating mobile station although specified by the originating mobile station, or a mobile station not specified by the originating mobile station, and the like.
Specifically, in such a group call, the originating mobile so station transmits, to the SIP server, information for inviting the non-participating mobile station to participate in the group call halfway. The SIP server transmits to the SMS server a request for transmitting the SMS information that requests a login to the SIP server. Subsequently, the SMS server transmits the SMS information that requests a login to the SIP server to the non-participating mobile station.    Non-patent literature 1: “SIP: Session Initiation Protocol,” RFC3261